By Kas “Rev ‘Em Up” Thomas

June 1994

EDITOR: After reading this article, auto conversions may be back on the powerplant selection list?

Will your engine make it to TBO (the manufacturer’s recommended Time Between Overhauls)? Every plane owner has wondered about this from time to time. Most owners are confident that their planes’ engines will “go the distance.” And many factors enter into the equation.

But the plain truth of the matter is some engines are more likely to go to TBO than others, because engine designers have (in some cases) stacked the cards either in favor of, or against, particular engines by virtue of critical design choices involving things like piston or valve metallurgy, compression ratio, rpm, turbo system layout, intercooling vs. no intercooling, etc.

Some engines have been blessed with extraordinarily generous TBO’s by engineering-blind marketing savants (Continental’s Malibu engine, with its ridiculously optimistic 2,000-hour TBO, comes to mind). Others have languished in the dim back-rooms of history, only to keep their original low TBO numbers (600 hours, for some helicopter engines). Every engine family is different.

And engine families, like human ones, definitely have their outcasts. Herewith, an admittedly subjective appraisal of the “TBO-worthiness” of the 30-or-so most popular engines flying today (with a look, also, at the TBO potential of some of aviation’s most memorable “orphan” engines).

Continental

O-200-A: This venerable 100-hp four-banger (an outgrowth of the C-85 and C-90) is very much worthy of the 1,800-hr factory TBO, and probably deserves a 2,000-hr TBO when using new Millennium cylinders. (Also, when you use Superior’s Millennium cylinders, you get to reset your timing to 28, which is good for a few extra horsepower).

If the O-200 has a vice, other than the stupid sprag­ clutch starter adapter (which fails unpredictably and costs $800 to replace), it’s valve sticking. Many owners use Marvel Mystery Oil (which is not FAA-approved) or Lenckite AVBlend (which is) to prevent stuck valves.

O-300-D: The O-300 (essentially a six-cylinder O-200) is found on a surprising number of Cessna 170 and 172 aircraft and tends to make the 1,800-hour TBO just fine, although not without an occasional bout of valve sticking, high oil temperature, or cylinder overheating.

Unfortunately, few big-name overhaul companies like to overhaul the O-300, it seems, supposedly because it’s a hard engine to make money on. TBO-busters should watch out for valve float caused by weak springs.

IO-360 series: Several thousand of these engines are flying around on Maules, Skymasters, and converted Stinsons. As sixes go, it’s a pretty viceless engine. Most versions have a 1,500-hr TBO; the IO-360-KB used in the Cessna 172XP (or USAF T-41) comes with a 2,000-hr TBO.

Over the years there have been problems with alternator drives, tachometer drives, oil pumps, and con-rod breakage, but all of these problems seem to have been worked out now and most owners can expect to make TBO with no trouble other than possible valve necking at high time.

TSIO-360-A, -B, -C: The turbo­ charged Skymaster engines were accorded 1,400-hr TBO’s by Continental and never got an increase, despite relatively trouble­ free automatic wastegate setups, AiResearch turbos, and modest take-off manifold pressure.

Of the three, the 225-hp -C model (used on the P-Skymaster) is definitely the one least likely to make TBO. Typical complaints include case cracking and piston burning.

TSIO-360-E, -F, -G, -K: Somehow, these fixed-wastegate Rajay-turbocharged models ended up getting 1,800-hr TBO’s, and the Skymaster engines didn’t. Go figure. Not good candidates for TBO-busting, even with aftermarket-designed intercooling kits.

Expect possible prob­lems with turbo housing cracks, oil leaks, low compression, burned pistons and valves, fuel pumps not staying calibrated, hairline cracks in rocker box area, and head-to-barrel separation problems.

O-470-R: Certainly one of the fin­est six-bangers around. Along with Amelia’s whereabouts, one of the big­gest remaining mysteries in aviation is why Continental has con­tinued to saddle O-470 owners with a measly 1,500-hour TBO (when such dastardly engines as the TSIO-520-BE and TSIO-520-AE get 2,000-hour TBO’s).

The O-470-R is cheaper to overhaul than a Lycoming IO-360-C1C6 and runs well on auto fuel, to boot. Typical problems are high oil usage and exhaust valve wear after 1,000 hours. Be sure to insist on .008-in. of cylinder choke (taper) at overhaul time and O-470-S pistons if you want to avoid chasing oil-consumption and compression problems (the -R can legally be converted to an -S at overhaul time; ask your overhaul agent).

O-470-U: Now here’s a silly deal, if you ask us. The O-470-U is potentially a 2,000-hr TBO engine. But to get your TBO officially raised from 1,500 to 2,000 hours, you need to do the following at overhaul time: install P/N 646267A2 or subsequent cylinder and valve assembly, P/N 648029 piston, P/N 649226 ring set, P/N 646277 lifter in the exhaust posi­tion, P/N 643779 oil pump, P/N 643749 oil pump gasket, P/N 649923 oil filter and two each P/N 402129P003 studs (or Cessna supplied oil filter adapter and associated parts) “and any sub-sequential part number of these parts.”

(See Conti­nental Service Bulletin No. M91-8.) All of these items will fit an O-470-R but won’t give the -R owner a TBO increase. (You have to have the lower-rpm, higher-compression -U engine). None of these parts is any better, in our opinion, than the stock parts; in fact, it looks an awful lot like Continental just wants to enforce the purchase of extra parts at overhaul time.

And the -U engine certainly is not intrinsically better (whether fortified with the foregoing parts or not) than any O-470-R. Why the TBO difference, then? We say it’s all fiction. Given a choice between the O-470-R and the non-auto-gas­ compatible O-470-U, we’d pick the -R.

IO-470 series: Delightful engines, now that four-ring pistons are available for all models. The bottom end is essentially the same as on any sand-cast IO-520; the top end is stout, and with high-compression pistons there is minimal lead accumulation.

Expect the usual big-Continental problems at high time, such as leakage past the exhaust valves, rocker shaft wear, wandering compression, and oil consumption. The 1,500-hr TBO is, if anything, too low.

IO-520-B, -C, -M: These Permold-crankcase ‘520’ models (Bonanza, Baron, 310) still suffer from some case crack­ing, although not as bad as in years past. Also, the series has been wrongly accused of having weak cranks.

In truth, the later-series 285-hp IO-520 engines with heavy case, heavy rods, thick-journal crank, and beefy Oberdorfer cylinder heads can be considered among the best, most / durable engines in aviation. The a 1,700-hr TBO is probably deserving of another increase, to 2,000 hours.

IO-520-F, -L: The 300-hp sand-cast­ crankcase 10-520 engines (Cessna 206, 210) are widely thought of as more du­rable than the Permold series (see above), but this is a myth. The 300-horse engines have their share of crankshaft fail­ure, case cracking, and unscheduled cylinder removals, and seem to throw more prop blades, too, for some reason (if the SDR database is any indication).

The fac­tory TBO of 1,700 hours is probably a tad optimistic. Expect all the usual big-Continental woes at high time, plus case fretting and main bearing wear.

IO-550 series: The 1,700-hr TBO is realistic. Expect the usual big-Continental fussiness in terms of valve and guide wear at high time. Some people say it’s a hot-running engine, but all the owners to whom we have talked are happy.

TSIO-520-B: The best of the turbo-520 Continentals (used on the T310), this 285-hp engine features an automatic-controller-regulated AiResearch TE06 turbo which provides a relatively mild redline manifold pressure of 32 inches. The 1,400-hr TBO ought to be increased to 2,000. (We’ve known some owners to go to 2,200 hours.)

TSIO-520-NB: Continental was virtually forced (by Cessna) to add intercooling to the 285-hp TSIO-520-K after Jack Riley showed the obvious ad­ vantages of doing it. Hence the 310-hp – NB, which is used in Cessna 340A and 414-series aircraft (and certain modi­fied Mooneys).

Early engines with the 1,400-hr TBO can be converted to a 1,600-hr TBO by following the latest Parts Catalog info. A rock-solid per­ former. Problems with high temps or wandering oil consumption are rare, compared to other 310-hp engines, and overhaul costs are $6,000 to $10,000 less than for most Navajo engines.

TSIO-520-LB, -WB: The P- Baron and 58TC engines (325-hp in the -WB variation) are expensive to over­ haul because they are the only Continental engines to use Bendix altitude-compensated fuel injection. Probably no other Continental engine fares as poorly for crankcase cracking. The official TBO of 1,600 hours is optimistic.

TSIO-520-BE: The Malibu engine is still a problem child, in our opinion, and the TBO of 2,000 hours remains as laughably unrealistic now as it was when the Malibu first came out in 1984. Real-world TBO 1,200 to 1,500 hours (and figure on some cylinder removals mid­ way).

GTSIO-520 series: Dozens of incremental im­provements (made over a period of 25 years) have made it possible, along with better pilot education, for operators to actually make it to TBO now with a geared Continental.

The official TBO for most of these en­gines is still 1,200 hours, but -L, -M, and -N versions are eligible for 1,600 hours with appropriate parts up­ grades during major over­ haul.

With good mainte­nance and proper operating technique, a fully modernized GTSIO-520 can easily make the 1,600-hr TBO. Older, doggy engines still have trouble making it to 1,200 hours. Plan on throwing most or all of your cylinders away at TBO.

Lycoming

O-235-L2C: The Tomahawk/152 en­gine is prone to massive lead deposits (in spark plugs and combustion-chamber crevices), but is otherwise a fine engine, eminently worthy of the 2,400-hr factory TBO when using LW-18729 pistons. Achilles’ heel is push rod mush­rooming due to solid-lifter design. Pis­ton breakage was also a problem until Lycoming came out with the long-life (LW-18729) piston.

The -L2C can be reworked to the -N2C (lead-buildup-resistant) con­figuration by modification of the cylinder head, albeit at the loss of two horse­ power. Vigorous leaning and constant use of TCP will keep most owners out of trouble. (Little-known fact… Prop strikes are common in this engine, due to the minuscule ground clearance on the Piper Tomahawk with a flat nose strut).

O-320-E2D: This is the only one of the 0-320 family to use 0-235 main bear­ings. A good, stout engine, worthy of the 2,000-hr TBO as long as exhaust valves are watched carefully for burning, thin­ing, and wobbling (S.B. 388B); does well on auto gas.

O-320-D2J: As with all O-320’s, you have to watch out for valve sticking; compliance with S.B. 388B every 400 hours is a must. Otherwise a very good engine.

O-320-H2AD: Owners of this engine need to pull and inspect lifters periodi­cally to catch incipient cam spalling. (Usually the lifters go first. And usually, it’s the furthest-forward ones on the in­ take side.) Not every owner sees cam problems, however; in fact, most don’t. Many of these engines go to 2,400 hours (400 past TBO) with no problem.

IO-360-A, -C, -D, -J: The 200-hp four-bangers are highly-loaded, somewhat vibration-prone engines with a tendency to use a quart of oil every four to five hours. TBO varies from 1,200 hours (on early models with small bearing dowels) to 2,000 hours on later models with improved camshafts and big bearing dowels; consult the latest editions of Service Bulletin No. 326 and Service Instruction No. 1263. Valve sticking, crankcase cracking, and high oil temps can be a problem.

IO-360-B, -E, -F: The parallel-valve design of these engines causes them to develop 180 horse­ power instead of 200. The lower rating is probably beneficial, though. By all accounts, most of these en­gines make TBO with no problem.

O-540-B: The 235-hp (parallel-valve) O-540 family is one of the strongest, most TBO-worthy engine families in all of aviation. Low compression ratio makes for potentially worrisome lead fouling problems on 100LL. (Either use TCP in your fuel or switch to autogas.) Otherwise a viceless engine.

O-540-L3C5D, -J3C5D: These en­gines (used on the 182RG and Turbo 182) have an integral accessory case and are stuck with the infamous Bendix dual mag, but are otherwise viceless (if you can put up with occasional valve sticking). Tur­bocharged variants use a tiny AiResearch TA04 turbo with throttle/wastegate in­terconnect. It’s a better interconnect scheme than the one used with the TIO-540-S, however (see below).

IO-540-A, -B: The top-exhaust 290-hp models (used on Aero Command­ers) are stuck with a 1,400-hour TBO, and are expensive to overhaul. (Cross-flow cylinders are expensive). Watch for the usual “big Lycoming” troubles in terms of case cracking, cylinder cracking, valve sticking, etc.

IO-540-D, -N: Bendix-fuel-injected 260-hp models (used on the Comanches) give good service and can generally be counted on to go to the 2,000-hr TBO.

IO-540-S, -AA: These high-compression-ratio brutes, used on the Aerostar 601 series, do not fare well under the cockamamie Rajay turbocharging scheme dreamed up by Ted Smith. Operate them well on the rich side of peak EGT, however, and you just may make it to the 1,800-hr TBO without burning holes in any pistons.

TIO-540-C1A: Density controllers and somewhat rare (for a TIO-540) parallel-valve cylinders make this an expensive engine to overhaul, on a dollar-per-horsepower basis. (This is a 250-hp engine, used on the Turbo Aztec.)

Run it on the rich side, and throttle back to 65%, if you want to make the 2,000-hr TBO with no problems. Engines without big main bearing dowels are limited to 1,500 hours; see S.I. 1225.

TIO-540-J2BD: The Piper Chieftain develops 43 inches MP (and 2,575 rpm) on takeoff about as “over-square” an en­gine as you’ll find in gen av. And you pay for it, not only in cracked cylinder heads and popped valve seats but high oil consumption, cracked crankcases, and magneto maintenance on the pressurized Bendix “dual” mag.

Lack of intercooling puts this engine very close to detonation limits in lean-cruise con­figuration. Plan on plenty of maintenance en-route to the 1,600-hr TBO. Typical warranty comebacks include wandering MP, low oil pressure, high oil temp, high oil consumption.

TIO-540-S1AD: Although it pulls only 36 inches on take-off, the Turbo Lance engine suffers from an outrageously bad up-flow cooling scheme (which works okay for the Chieftain, but is poorly implemented in the PA-32) and a me­chanical interconnect between throttle and wastegate that makes no sense at all.

The exhaust system became signifi­cantly less nightmarish in 1993 when Lycoming introduced a single-piece cross­ over pipe (see Service Bulletin No. 499). Many owners have added intercooling in an attempt to correct high temps. (There is no way around the Bendix dual mag, alas).

TIO-541-E1C4: The Duke engine was designed to run all day long at FL 250, and for the most part it’s a dandy way to achieve 380 horsepower (albeit minus intercooling). Early versions had a hard time reaching the 1,200-hour TBO.

Later engines incorporating the improved crank­ cases and cylinder assemblies described in the latest revisions to Service Bulletin Nos. 334 and 353 can reasonably expect to make the 1,600-hr TBO if operated with regularity.

Seldom-flown engines tend to get short camshaft life, even with the Lycoming oil additive (see S.B. 318). Thanks to poor parts commonality with the rest of the Lycoming line, you can expect to pay upwards of $35,000 at over­ haul time.

TIGO-541-E1A: You had to ask! Well believe it or not, many operators actu­ally make it to the 1,200-hr TBO without entering Chapter 11, but not without the usual geared-engine travails. Don’t try to economize on fuel flow, though.

IO-720-A: This engine is basically two IO-360’s on a common shaft. All top-end components are in plentiful supply, but you’d better pray that your crank doesn’t flunk Magnaflux testing at over­ haul; also, crankcases are in short sup­ply.

(While we’re talking about bottom­ end components, the IO-720’s long, spindly camshaft usually is not reusable at overhaul time; with that many lobes, at least one of them is bound to be bad. Figure on $1,700 to replace the cam.) But hey, you want 400 ponies under the hood? You gotta pay the Piper.

Orphans

Note: Engines in this category are essentially antiques, out of production for at least 25 years, and shunned by most overhaul companies (not to mention the factories).

E-185 & E-225: These predeces­sors of the O-470 were some of Continental’s best postwar designs. They’re still great engines, providing you don’t mind chasing the occasional low- oil-pressure or high-oil-temperature problem. (The availability of late-style four-ring pistons has done away with oil burning, for the most part.)

At overhaul time, expect to look high and low (and spend big bucks) for main bearings, hydraulic lifters, and oil pump parts. And God help you if you need the accessory case rebushed. (Accessory case work alone has cost many an owner $700.) Thankfully, top-end parts are plentiful, and the 1,500- hr TBO is realistic.

GO-300 series: Cessna 175 owners have suffered a bad rap for years over this geared variant of the Continental 0-300, but really, it’s a good engine (cer­tainly deserving of a higher TBO than the 1,200 hours suggested by the fac­tory). Crankcases are getting hard to find, so beware of case cracking.

Typical complaints are high CHT, high oil temp, valve sticking, and cracked cylinder heads. Expect to be turned away by most engine shops, who see the bottom end as a liability. Top-end parts are plentiful.

IO-346: Continental built just over a thousand of these four-cylinder, 165- hp engines (with top-end components similar to those found on an IO-520) for the Beech A23 Musketeer, and there are still dozens flying. The 1,500-hr TBO may be realistic, but if you can’t find parts at overhaul time, who cares?

TSIO-520-AE: Technically, this en­gine (for the Cessna T303) isn’t an or­phan just yet. But we expect it to be soon. Continental sold only 650 engines to Cessna before Crusader production was terminated in 1984; no airframe manufacturer has bought a new one since.

In case you don’t recall, this engine was a one-off, clean-sheet-of-paper redesign of the TSIO-520 aimed at chop­ping excess pounds off the Permold core. (To get the weight down, Continental cut the cylinder fin area by about half, hollowed out the camshaft, reduced sump volume to eight quarts, and put light­ening holes in every gear and flange.)

The end result? A 250-hp TSIO-520 that has almost no parts commonality with other members of the TSIO-520 family. (Bear in mind, too, that half of these engines are counter-rotating.) The 2,000-hr TBO was never taken seriously by anyone outside of Mobile.

O-290-D: Lycoming’s ex-ground­ power-unit motor is probably William­ sport’s best-known orphan. The 125-hp version, used in thousands of Super Cubs, Pacers, and Tripacers, carries a 2,000-hr TBO; the 135-hp -D2 variant was built in much larger quantity and carries a 1,500-hr TBO.

As with most orphans, top-end parts are in good supply; bottom­ end parts are another story. Even so, the salvage and surplus markets will probably keep these solid four-bangers going well into the next century. All in all, as good an orphan as you’ll find. Many vertical models (VO-435, TVO-435) are still flying on Bell helicopters.

GO-435: Surprise! Another great engine. The 260-hp GO-435-C2, intro­duced with the Navion B, turns out to be just a six-cylinder 0-290 (same top end parts), eminently worthy of the 1,200-hr TBO. About 110 Twin Bonanzas were produced with this engine. Many are still flying.

O-340: No misprint! Lycoming ac­tually churned out a few 340-cubic-inch four-bangers for Jack Riley in the Twin Navion days. It is not known whether any are still flying. (TBO: 2,000 hrs).

IO-360-B1A: This Lycoming is found on 1961-62 Beech B95A Travel Airs. What makes it an orphan is the Simmond Pre­cision fuel-injection system, which is essentially obsolescent and terrifyingly expensive to overhaul. Most other Lycomings use Bendix fuel-injection. Buyer beware!

GO-480: Although the planetary reduction gearing on this 295-hp engine (used on the 560E Commanders and D50 Twin Bonanzas) cannot legally be worked on by just any overhauler, the core en­gine is little more than a six-cylinder O-360 with Bendix pressure-carburetion. It’s a shame the TBO is only 1,400 hours. T-Bone owners with this engine are lucky.

GSO-480, IGSO-480: Although sharing the Lycoming GO-480’s 1,400-hr TBO, these engines produce more power (340 horses) and are considerably more expensive to overhaul, thanks to mechanical supercharging and (in the IGSO-480) fuel injection. Watch out for hideously expensive Simmonds injection system in the G50, H50, and J50 Twin Bonanza’s IGSO-480-A1A6. (Other IGSO – engines are Bendix-injected).

IGO-540 series: Like the ‘480’ engines, the geared 540’s turn 3,400 rpm for take-off and require the blessing of the Pope before the pinion cage comes off for repair. TBO on this particular oddball series was and is 1,200 hours. Watch out for obsolete accessories such as low-tension ignition system. (Lycoming’s IGO-series was used on 560F Commanders.)

IGSO-540 series: 1,200-hr TBO is realistic, since gear reduction cage invariably needs attention by then. (That reduction unit has to deal with 380 horsepower on take-off). The -BIA has Simmonds injection; others have Bendix. Figure on spending a bundle at over­ haul time.

TIO-540-A1A: Used only on the M22 Mooney Mustang. Has the distinction of being aviation’s only en­gine with a 1,300-hr TBO. IVO-540-A1A: Probably the ultimate oddball engine, unless you want to talk about the 270-hp supercharged Continental that powered Cessna’s ill-fated Skyhook helicopter. The Lycoming IVO-540 was used in Brantly’s Model 305 five-passenger helicopter; it carries the shortest factory TBO of any engine, at 600 hours.